A battery pack having a secondary battery and a controlling and computing portion for controlling charging and discharging of the secondary battery is widely used as a power supply portion attached to an electric device of a note-type personal computer (PC) or the like. The secondary battery of the battery pack used in the electric device is charged with an external power supply (commercial power supply) through a power supply portion of the electric device, and when the electric device is not connected to the external power supply, instead of the external power supply, the secondary battery supplies power to the electric device.
For example, properties of a lithium ion battery as the secondary battery, concretely a charge capacity and a cycle life widely change depending on charging voltages as shown in FIG. 7. When a charge end voltage for detecting charging end is set at a high value to charge the secondary battery (the lithium ion battery), the large charge capacity is obtained as shown in the properties of A in FIG. 7, but the cycle life is short. In contrast, when the charge end voltage is set at a low value to charge the secondary battery (the lithium ion battery), the charge capacity is decreased as shown in the properties of B in FIG. 7, but the cycle life is long.
By using such battery properties, it is thought that the battery pack is used selectively in the high capacity mode or the long life mode by changing the charge end voltage at charging time of the secondary battery (for example, refer to patent literature 1). Concretely, the charge end voltage of the secondary battery (the lithium ion battery) is set at 4.2 V/cell in the high capacity mode, and the charge end voltage of the secondary battery (the lithium ion battery) is set at 4.0 V/cell at the long life mode, and then the secondary battery is charged.